Nitride containing ductile steel



Nov. 3, 1964 HAJIME NAKAMURA NITRIDEI CONTAINING DUCTILE STEEL FiledJan. 4, 1962 7 Sheets-Sheet l T 1 a 1 VNQTCH CHARPY IMPACT VALUE KILLEDSTEEL TEST TEMPERA'IUREC 1 2| V NOTCH CHARPY IMPACT VALUE TESTTEMPERATURE C INVENTOR HAJIME NAKAMURA Y ATTO R N EY 3, 1964 HAJIMENAKAMURA 3, 5, 5

NITRIDE CONTAINING DUCTILE STEEL Filed Jan. 4, 1962 7 Sheets-Sheet 2 Tij- ASTM FERRITE GRAIN SIZE NUMBER N O 1 I I l x l O I 2 3 ASTM FERRITEGRAIN SIZE MJMBER CI 0 Q05 01 m5 AMOUNT OF METALLIC ALUMINUM IN SOLIDSOLUTION%) INVENTOR HAJIME NAKAMURA I l l ATTORNEY Nov. 3, 1964 HAJIMENAKAMURA 3,155,495

NITRIDE CONTAINING DUCTILE STEEL v '7 Sheets-Sheet 3 Filed Jan. 4, 1962NUMBER OF FERRlTE GRAIN IN 1mm AMOUNT OF METAL ALUMINUM IN SOLIDSOLUTION (0.003- 015%) AMOUNT OF METAL ALUMlNUM lN SOLID SOLUTION (3.5-0.36%)

m B 6 4 mum-E32 mN w 23mm mtmmwm 2.54

w 0 m 0 w 0 7 w 6 O. O mm A 4 O. 0

INVENTOR HAJIME NAKAMURA ATTO R N EY Nov. 3, 1964 HAJIME NAKAMURA 9 3NITRIDE conmmmc DUCTILE STEEL 7 Sheets-Sheet 4 Filed Jan. 4, 1962 Photo2 xuoo The same as in Photo 1 Photo 1 IELR o 3% Nital C8, 930C 1 hrair-cooled ASTM ferrite grain size 310. 9.5

Photo 1 x1400 The same as in Photo 3 Photo 3 C8, 930C 1 hr oil-cooled656C 1.5 hrs air-cooled ASTM ferrite grain size No. 11

INVEN TOR. HA JIME NAKA I MR1:

Attorneys Nov. 3, 1964 HAJIME NAKAMURA 3,155,495

NITRIDE CONTAINING DUCTILE STEEL.

Filed Jan. 4, 1962 '7 Sheedas-Sheet 5 Photo 5 x100 3% Nital The same asin Photo 7 No. 80, 930C 1 hr oil-cooled 933- 0 1 hr oil-cooled 656C 1.5hrs air-cooled 650C 1.5 hrs air-cooled Austenite grain size No. 8

No. 85, 930C 1 hr oil-cooled The same as in Photo 5 650C 1.5 hrsair-cooled 930C 1 hr oil-cooled 650C 1.5 hrs air-cooled Austenite grainsize No. 6

INVENTO HAJII-E NAKAMURA Attorneys Nov. 3, 1964 HAJIME NAKAMURA3,155,495

rumors. commune DUCTILE STEEL '7 Sheets-Sheet 6 Filed Jan. 4, 1962 Photb9 C8, Each-action Replica.

Photo 10 m1, Extraction Replica INVENTOR. HAJIME NAKAMURA Attorneys 3,1964 HAJIME NAKAMURA 3,155,495

NITRIDE CONTAINING DUCTILE STEEL 7 Sheets-Sheet 7 I Filed Jan. 4, 1962Photo 11 No. 85, Extraction Replica Photo 12 No 80, Extraction ReplicaINVENTOR. mmaz NAKAMURA flllh 3 Attnrneys United States Patent C) M3,155,495 NHRIDE CGNTAINENG BUCTELE STEEL Haiime Nalmmnra, Tokyo-to,Japan, assignor to lishikawajima-Harima .luhogyo Kahushilsi Kaislia,Tokyoto, lapan, a company of Japan Filed Jan. 4, 1962, Ser. No. 16 2 201Claims priority, application .lapan, Mar. 11, 1961, Tao/8,484- 2 Claims.(Cl. 75-124) The present invention relates to a nitride containingductile steel.

In the construction of structures in services at temperatures lower thannormal environmental temperature, such as in the case of ships hulls orpressure vessels, steels have been hitherto used, which were deoxidizedby both silicon and aluminum, because the notch-transition temperaturesof such steels are substantially lower than those that are deoxidized bysilicon only; that is to say, steels of former kind are tougher, or moreductile, at lower emperatures than the latter. Although the main reasonfor the use of such deoxidizers as silicon and aluminum and theattendant improvement of transition temperature characteristics isthought to be due to Le action of aluminum oxide and aluminum nitride torefine and uniformize the ferrite grains. The present invention containsevidence of the discovery that it is the precipitated aluminum nitridewhich contributes to the desired characteristics more effectively.

In producing steel containing precipitated aluminum nitride, however, itis almost impossible to produce a steel containing only aluminum nitrideand, inevitably, a certain quantity of metallic aluminum is dissolved asin solid solution. The present invention is based on the discovery thatit is impossible to get a fine grained steel, and therefore one which isductile at low temperatures, if more than a certain quantity of metallicaluminum is contained in solid solution.

It is an object of the present invention to obtain ductile carbon-steelsand low-alloy steels that have uniform and very refined grains bymaintaining a quantity of precipitated aluminum nitride and dissolvedmetallic aluminum in the steel; and the present invention relates toductile steels containing 0.040.25% carbon, 0.010.50% silicon, 1.00% orless than 1.00% manganese, less than 0.035% phosphorus and sulphur,0.030.l2% aluminum nitride as precipitated and 0.003-0.15% metallicaluminum in solid solution and having a ferrite grain size of more thanN0. 9 ASTM and a V notch Charpy impact value of more than 15 kg-rn/cm.at C.

Reference is made to the accompanying drawing and photos, wherein:

FIGURE 1 shows V notch Charpy-transition temperature curves of variouskinds of carbon steel containing different amounts of precipitatedaluminum nitride and metallic aluminum in solid solution. FIGURE 2 showsV notch Charpy-transition temperature curves of various kinds oflow-alloy steels containing different amounts of precipitated aluminumnitride and metallic aluminum in solid solution. FIGURE 3 shows therelation between the quantity of metallic aluminum in solid solutionandferrite grain size. FIGURE 4 shows the relation between the condition ofheat treatment and ferrite grain size in various types of steelcontaining less than 0.15% metallic aluminum in solid solution. FIGURE 5shows the relation between the quantity of metallic aluminum in solidsolution and the number of ferrite grains in '1 mmfi. FIGURE 6 shows therelation between the quantity of precipitated aluminum nitride and theferrite grain size in various types of steels contain ng differentquantities of metallic aluminum in solid solution. Photo 1 and Photo 2are photomicrographs showing the ferrite grain 3,l55,d35 Fatented Nov.3, 1964 size when a carbon steel containing 0.09% carbon, 0.091%precipitated aluminum nitride and 0.0024% metallic aluminum is heated at930 C. for one hour and then air-cooled. Photo 3 and Photo 4 arephotomicrographs showing the ferrite grain size when the same type ofsteel as used in Photo 1 and Photo 2 is heated at 930 C. for one hourand oil-cooled, and reheated at 650 C. for 1.5 hours then air-cooled.Photos 5-8 are photomicrographs showing the austenite grain size ofvarious types of lowalloy steels containing different quantities ofprecipitated aluminum nitride and metallic aluminum in solid solution.Photo 9 and Photo 10 are electron-micrographs showing the precipitatedaluminum nitride in carbon steel. Photo 11 and Photo 12 areelectronmicrographs showing the precipitated aluminum nitride in alow-alloy steel.

Table 1 shows the chemical compositions of some of the sample carbonsteels used in the tests conducted.

Sample No. C Si Mn Ni Cr Cu Total Total Al N3 Sample No. AlN Met; Acid AHeat A1 Sol. Al Treatment. 7

Commercial killed steel 0.0014 As rolled. 0. 024 0. 034 0. 037 930 C/H.0.096 0.156 0. 017 930 C/H. 1.073 1. 092 0. 032 930 (31H. 2. 657 2. 6720. 026 930 O/H.

FIGURE 1 shows the transition temperature curves of the carbon steelsshown in Table 1. The V notch Charpy-transition temperature curves ofsteels A41 and A51, which contain more metallic aluminum, are flatterthan that of commercial killed steel which has a lower aluminum content,while the V notch Charpy-transition temperature curve of C8, containingless metallic aluminum in solid solution, considering the amount ofaluminum nitride, runs steeper, and the curve of C8 which was heattreated (as to be explained later) runs even higher. The reason why theV notch Charpy-transition temperature curve of All proceeds less steepthan that of C8, in spite of the fact that All and C8 contain the samequantity of aluminum nitride, is that All contains more metallicaluminum in solid solution. The result shown in FIGURE 1 is mainly dueto the ferrite grain size, and

it is to be seen that the coarser the ferrite grain size, the

higher is the transition temperature.

FIGURE 3 and FIGURE 4 show the change in ferrite grain size as afunction of the concentration of metallic aluminum in Solid solution forcarbon steels with 0.04- 0.25% carbon. It is observed that, in order toget finer grains than ferrite grain size ASTM No. 9, the top limit ofthe dissolved aluminum is'0.1% for a heat treatment of heating at 930 C.for one hour and air-cooling therefrom (N); the limit is 0.36% for aheat treatment consisted of heating at 930 C. for one hour, oil-coolingtherefrom, re-heating at 650 C. for 1.5 hours and aircooling therefrom(QT). Although the relationship be tween the ferrite grain size and thecarbon content is not clearly definable, because, as the carbon contentis increased, the amount of gaseous nitrogen whichcan be dissolved inthe metal matrix, and hence the yield of aluminum nitride, is decreasedthus tempting an increase of the concentration of dissolved metallicaluminum, the

graphs shown in FIGURES 3 and 4 represent relations with all these abovementioned factors considered.

The relations indicated in FIGURE 3 and FIGURE 4 are reproduced on alog-log scale in FIGURE 5, showing a linear relationship.

FIGURE 6 shows the relation between precipitated aluminum nitride andferrite grain size in the case of various types of steel containingditferent amounts of metallic aluminum in solid solution. Namely, in thecase when the concentration of metallic aluminum in solid solution issmall, the lower limit of the amount of precipitated aluminum nitridenecessary for a ferrite grain size of ASTM No. 9 is above 0.035% whenthe steel is air-cooled after having been heated at 930 C. for 1 hour,and is above 0.0075% when the steel is oilcooled after having beenheated at 930 C. for one hour and air-cooled after having been heated at650 C. for 1.5 hours. And in the case the amount of the metallicaluminum in solid solution is large, the limit of the amount ofprecipitated aluminum nitride necessary for a ferrite grain size of ASTMNo. 9 is more than 0.1% when Table 3.Meclzanical FIGURE 2 shows V notchCharpy-transition temperature curves of such low-alloy steels of hightensile strength. Specimen No. 80 containing metallic aluminum in solidsolution rather plentifully as compared with the aluminum nitride, N0.85 contains a large amount of aluminum nitride along with much ofmetallic aluminum in solid solution, and No. 82 the composition of whichis intermediate between 80 and 85, are found to have different impactvalues at low temperatures. Especially No. 85 is found to have excellenttoughness, in spite of its high tensile strength. Thus, it is to beconsidered demonstrated that an identical tendency is held for hightensile strength low-alloy steels as for carbon steels describedearlier.

Properties of Carbon Steel Conditions Diameter Gauge Yield TensileElongation, Reduction Rupture Yield No. of heat of tested length, point,strength, percent of area, position rate,

treatment pieces, mm. mm. leg/r11. kg./rn. percent percent NorE.N; 9300., 1 hour, air-cooled. 61.1; 930 (3., 1 hour, oil-cooling and 650 0.,1.5 hours, air-cooled.

Table 4.-Mechanzcal Properties of Low-Alloy Steel Conditions DiameterGauge Yield Tensile Elongation, Reduction Rupture Yield No. of heat oftested length, point, strength, percent 01 area, position rate,

treatment pieces, mm. mm. kgJrn. kg./m. percent percent N OTE.-Q.T; 9300., 1 hour, oil-cooled and 650 0., 1.5 hours, air-cooled.

the steel is air-cooled after having been heated at 930 C. for one hour,and is more than 0.0575 when the steel is oil-cooled after having beenheated at 930 C. 50 for one hour and air-cooled after having been heatedat 650 C. for 1.5 hours.

From FIGURES 36, and considering the cooling rate during the heattreatment employed in the experiments, it is evident that a fine grainedsteel can always be obtained by maintaining the amount of precipitatedaluminum nitride between 0.030.12%, and the amount of metallic aluminumin solid solution at 0.0030.15%, respectively.

When the test results obtained on carbon steels were applied tolow-alloy high tensile strength steel, the same tendency was seen as inthe cases of carbon steel mentioned above. Table 2 below shows thechemical composition of a few samples chosen from various types oflow-alloy steels with high tensile strength used in the test.

Table 2.-Clzemical Composition of Various Type of Low-Alloy Steel It ispossible to make ferrite grain size finer by a degree of 14 ferritegrain size number than that of the as-rolled or as-forged state, ifcarbon steels and lowalloy steels of this invention are quenched andquenchtempered from a temperature higher than the transformationtemperature of the steel where the precipitation of aluminum nitride isvigorous. And, moreover, it is possible to make ferrite grain size muchmore finer by a degree of 11.5 ferrite grain size number, by changingthe method of cooling from air-cooling to oil-cooling or water-cooling.At the same time, it was found that the transition temperature can beimproved by shifting it towards lower values. Photos 1-4 show an exampleof this, demonstrating clearly that the ferrite grain size being refinedby a degree of 1-1.5 in ferrite grain size number. Photos 9-12 areelectron-micrographs showing precipitation conditions of aluminumnitride, this effect playing a most important role in this invention, invarious types of carbon steels and low-alloy steels. Photos 9 and 10 aremade from carbon steels, While Photos 11 and 12 are made of low-alloysteels. Photo 9 shows the case where the steel contains 0.091%precipitated aluminum nitride, and the square or rectangular blackportions in this photo are the aluminum nitride precipitated particles.Photo 10 is the case of specimen A41 which shows a lower precipitateconcentration of aluminum nitride and other non-metallic inclusions. inPhoto 11, in the case of specimen the black squares or rectangles arethe aluminum nitrides, and the round or rounded shapes are othernon-metallic inclusions, chiefly precipitated carbide. Photo 12 ofspecimen 80 shows little aluminum nitride. It is to be seen clearly thatthe grain size can be made even more finer and uniform thus helping toimprove the low temperature properties of the above mentioned aluminumnitride bearing steels with an amount of metallic aluminum in solidsolution not exceeding a certain limit, by distributing the aluminumnitride in a large quantity and evenly throughout the grains.

As explained heretofore, the nitride containing ductile steel of thisinvention that is made to contain 0.03-0.12% precipitated aluminumnitride and is also made to contain only 0.003-0.15% metallic aluminumin solid solution has a very remarkable high ductility at lowtemperatures, and exhibits excellent properties as steel for use notonly as a construction material in services at temperatures lower thannormal atmospheric temperature conditions but also as a steel that canbe used for various other structures.

What I claim is:

1. In a ductile steel composition consisting essentially of 0.040.25%carbon, 0.010.5% silicon, up to 1.0% manganese, not over 0.035% 0.035%sulphur, and 0.030.12% precipitated aluminum nitride, in addition to theiron content, the improvement which consists of metallic aluminum beingpresent in a concentration of 0.003-0.15%, the steel having anphosphorus, not over 2 ASTM ferrite grain size number higher than 9, anda 2 mm. V-notch Charpy impact value higher than 15 kg.- fir/cm. at 0 C.

2. In a ductile steel composition consisting essentially of 0.04-0.25%carbon, 0.0l-0.5% silicon, up to 1.0% manganese, less than 0.035%phosphorus, less than 0.035% sulphur, 0.03-0.l2% precipitated aluminumnitride, and at least one of the elements nickel, chromium, molybdenum,vanadium and copper from traces up to 1.0% and boron up to 0.1%, inaddition to the iron con tent, the improvement which consists ofmetallic aluminum being present in a concentration of 0.0030.15%, thesteel having a 2 mm. V-notch Charpy impact value higher than 10kg.-rn./cm. at 0 C.

References Cited in the file of this patent UNITED STATES PATENTS2,679,454 Offenhauer May 25, 1954 FOREIGN PATENTS 808,556 Great BritainFeb. 4, 1959 OTHER REFERENCES Journal of Research of the National Bureauof Standards, vol. 48, No. 3, March 1952. Research paper 2305. Pages 193to 199.

Bullens: Steel and Its Heat Treatment, 5th edition, vol. 1, pages 63 to66.

2. IN A DUCTILE STEEL COMPOSITION CONSISTING ESSENTIALLY OF 0.04-0.25%CARBON, 0.01-0.5% SILICON, UP TO 1.0% MANGANESE, LESS THAN 0.035%PHOSPHORUS, LESS THAN 0.035% SULPHUR, 0.03-0.12% PRECIPITATED ALUMINUMNITRIDE, AND AT LEAST ONE OF THE ELEMENTS NICKEL, CHROMIUM, MOLYBDENUM,VANADIUM AND COPPER FROM TRACES UP TO 1.0% AND BORON UP TO 0.1%, INADDITION TO THE IRON CONTENT, THE IMPROVEMENT WHICH CONSISTS OF METALLICALUMI-